This invention relates generally to step-and-repeat alignment and exposure systems for aligning each of an array of different regions of an object, such as a semiconductive wafer, with respect to an image of another object, such as a reticle, and for photometrically printing that image at each of those regions by employing a projection lens of the reduction type. More specifically, this invention relates to improved optical apparatus for use in such systems to facilitate utilization of the projection lens in aligning each region of the semiconductive wafer with respect to the image of the reticle.
In each of U.S. patent applications Ser. Nos. 289,790, 396,099, 278,402 and 388,147 improved step-and-repeat alignment and exposure systems are disclosed that may be employed for aligning each of an array of inchoate dice regions of a semiconductive wafer with respect to an image of a main reticle containing a level of microcircuitry to be photometrically printed at each of those inchoate dice regions. Each system includes a main stage movable along orthogonal axis of motion of the system, a substage mounted on the main stage for aligning a reference mark with respect to the orthogonal axes of motion of the system, a rotatable vacuum chuck mounted on the main stage for holding the semiconductive wafer, a reticle stage mounted above the main stage for holding main reticle, and a projection lens disposed between the main stage and the reticle stage and operable with an associated beam splitter and an associated objective lens unit for directly viewing an aerial image of the image of the main reticle and also of the reference mark or the semiconductive wafer depending on the position of the main stage.
Thus, while employing the projection lens and its associated split-field objective lens unit to directly view aerial image of the image of the main reticle and also of the reference mark, the main stage and the substage can be employed to align the reference mark with respect to the orthogonal axes of motion of the system during a reference mark set-up alignment operation, and the main stage and the reticle stage can thereafter be employed to align images of global alignment marks disposed on the main reticle (at opposite sides thereof) with respect to corresponding portions of the reference mark and, hence, with respect to the orthogonal axes of motion of the system during a main reticle alignment operation. Similarly, while subsequently employing the projection lens and its associated split-field objective lens unit to directly view the aerial image of the image of the main reticle and also of the semiconductive wafer, the main stage can be employed to alternately align global alignment marks disposed on the semiconductive wafer (at opposite sides thereof) with respect to the images of the global alignment marks of the main reticle to align the semiconductive wafer as a whole with respect to the orthogonal axes of motion of the system during a global wafer alignment operation, and thereafter to align local alignment marks disposed on the semiconductive wafer between selected ones of the inchoate dice regions of the semiconductive wafer with respect to an image of a corresponding local alignment mark on the main reticle to more precisely align each inchoate dice region with respect to the image of the main reticle during a precision region-by-region or local wafer alignment operation.
The projection lens is corrected for exposure light (namely, blue light having a wavelength of 436 nanometers) so that the projection lens can be employed to photometrically print the level of microcircuitry contained on the main reticle at each inchoate dice region of the semiconductive wafer, once that region is aligned with respect to the image of the main reticle, in precise alignment with other previously or yet-to-be printed levels of microcircuitry by exposing a photoresistive film on that region in accordance with that image during a wafer exposure operation. A controllable light source unit, an associated plurality of masking elements, another beam splitter, and an imaging lens disposed between that beam splitter and the main reticle are provided for selectively illuminating different portions of the main reticle with exposure light so that the global and local alignment marks of the semiconductive wafer can be illuminated with exposure light during the global and precision local wafer alignment operations performed with the projection lens and its associated split field objective lens unit without exposing the photoresistive film on each inchoate dice region. However, due to interference patterns that may be created for some photoresistive films or other surface conditions of the semiconductive wafer at the wavelength of the exposure light for which the projection lens is corrected, it is often desirable to employ a photoresistive film that is opaque to the exposure light. In this case the projection lens and its associated split field objective lens unit cannot be employed in performing the global and precision local wafer alignment operations.
As disclosed in U.S. patent application Ser. Nos. 289,790 and 396,099, the controllable light source unit can be adjusted to selectively illuminate the reticle with nonexposure light of longer wavelength (namely, green light having a wavelength of 546 nanometers), and a compensating lens can be moved into position to compensate the projection lens for the nonexposure light of longer wavelength during the global and precision local wafer alignment operations involving a photoresistive film that is opaque to the exposure light. However, a serious disadvantage of this technique is the loss of time in adjusting the controllable light source unit and moving the compensating lens into position for the global and precision local wafer alignment operations and thereafter readjusting the controllable light source unit and moving the compensating lens back out of position for the wafer exposure operation. Moreover, the image plane of the projection lens may be shifted by the compensating lens, thereby resulting in the possibility of serious alignment errors.
In U.S. patent application Ser. No. 289,790 an auxiliary optical unit is disclosed that may be employed in performing the global and precision local alignment operations when the projection lens and its associated split field objective lens unit cannot be so employed due, for example, to the use of a photoresistive film that is opaque to the exposure light. This auxiliary optical unit is disposed above the main stage and offset from the projection lens along one of the orthogonal axes of motion of the system. The auxiliary optical unit includes an adjustable auxiliary reticle containing an alignment mark, a source of white light for illuminating the auxiliary reticle, and an auxiliary objective lens unit for directly viewing an image of the alignment mark of the auxiliary reticle and also either the reference mark or the semiconductive wafer depending on the position of the main stage.
Thus, while employing the auxiliary objective lens unit to directly view the image of the alignment mark of the auxiliary reticle and also the reference mark, the main stage and the adjustable auxiliary reticle can be employed to align the alignment mark of the auxiliary reticle with respect to the reference mark and, hence, with respect to the orthogonal axes of motion of the system and thereby also measure the offset between the projection lens and the auxiliary objective lens unit during an auxiliary reticle set-up alignment operation. While thereafter employing the auxiliary objective lens unit to directly view the image of the alignment mark of the auxiliary reticle and also the semiconductive wafer, the main stage can be employed to align each global alignment mark of the semiconductive wafer with respect to the image of the alignment mark of the auxiliary reticle and, hence, with respect to the orthogonal axes of motion of the system during the global wafer alignment operation and subsequently to align each local alignment mark of the semiconductive wafer with respect to the image of the alignment mark of the auxiliary reticle during the precision local wafer alignment operation. The main stage can thereupon be moved to coordinate positions determined by the measured offset between the projection lens and the auxiliary objective lens unit so that the projection lens may be employed to photometrically print the level of microcircuitry contained on the main reticle at each inchoate dice region of the semiconductive wafer.
A serious disadvantage of such an auxiliary optical unit is the loss of time in having to move the main stage by the offset between the projection lens and the auxiliary objective lens unit to perform the global and precision local wafer alignment operations and the additional loss of time in having to move the stage by the same offset to perform the wafer exposure operation. Moreover, the possibility of alignment errors is increased due to the necessity of moving the stage by that offset between the wafer alignment and the wafer exposure operations.
Accordingly, it is the principal object of this invention to provide improved optical apparatus for enabling the projection lens of a step-and-repeat alignment and exposure system, such as those described above, to be employed in performing the global and precision local wafer alignment operations even when a photoresistive film opaque to exposure light of the wavelength for which the projection lens is corrected is disposed on each inchoate dice region of the semiconductive wafer.
Another object of this invention is to provide an improved step-and-repeat alignment and exposure system in which a projection lens, employed for photometrically printing a level of microcircuitry on a semiconductive wafer covered with a photoresistive film opaque to the wavelength of exposure light for which the projection lens is corrected, can be employed in performing the global and precision local wafer alignment operations, as well as the wafer exposure operation, without the necessity of employing a controllable light source unit for selectively illuminating different portions of the main reticle with the exposure light, without the necessity of adjusting the controllable light source unit to selectively illuminate the main reticle with nonexposure light of longer wavelength and concomitantly employing a compensating lens to compensate the projection lens for the nonexposure light, and without the necessity of moving the stage by an offset distance to perform the global and precision local wafer alignment operations or the wafer exposure operation.
Still another object of this invention is to provide an improved step-and-repeat alignment and exposure system as in the last object in which, like the system disclosed in U.S. patent application Ser. No. 388,147 the entire field viewed on the semiconductive wafer by an objective lens unit associated with the projection lens is illuminated by nonexposure light that is of longer wavelength than the exposure light, that does not pass through any reticle, and that does not illuminate the entire semiconductive wafer.
These and other objects, which will become apparent from a reading of this specification and an inspection of the accompanying drawings, are accomplished according to the illustrated preferred embodiment of this invention by employing a split-field objective lens unit, operable with a nonchromatic beam splitter, the projection lens, and a source of exposure light of a wavelength for which the projection lens is corrected, for directly viewing an aerial image of images of the global alignment marks of the main reticle and also of the corresponding portions of the reference mark to facilitate alignment of the images of those global alignment marks with respect to the reference mark during the main reticle alignment operation; by employing a first single-channel objective lens unit, operable with a chromatic beam splitter, the nonchromatic beam splitter, the projecting lens and the source of exposure light, for directly viewing an aerial image of an image of another alignment mark disposed on the main reticle between the global alignment marks and also of a corresponding portion of the reference mark to facilitate alignment of that corresponding portion of the reference mark with respect to the image of that other alignment mark of the main reticle; and by employing an adjustable alignment reticle and a second single-channel objective lens unit, operable with both the chromatic and nonchromatic beam splitters, the projection lens and a first source of nonexposure light of longer wavelength than the exposure light, for imaging an alignment mark of the alignment reticle at the image plane of the projection lens and for directly viewing an aerial image of the image of that alignment mark and also of a corresponding portion of the reference mark to facilitate adjustment of the alignment reticle to align the image of that alignment mark with respect to that corresponding portion of the reference mark and, hence, with respect to the image of the other alignment mark of the main reticle in a bore-sighting alignment reticle set-up alignment operation. This bore-sighting alignment reticle set-up alignment operation can be repeated as frequently as desired during the operation of the step-and-repeat alignment and exposure system to maintain the alignment accuracy of the system.
Following the bore-sighting alignment reticle set-up alignment operation, the second single-channel objective lens unit can be employed for directly viewing an aerial image of the image of the alignment mark of the alignment reticle and also of each global alignment and local alignment mark of the semiconductive wafer to facilitate alignment of each of those alignment marks of the semiconductive wafer with respect to the image of the alignment mark of the alignment reticle during the global and precision local wafer alignment operations. Additional optical apparatus, including an apertured field-stop plate and a second source of nonexposure light of the same wavelength as the first source of nonexposure light, is employed for illuminating the entire field viewed in the image plane of the projection lens by the second single-channel objective lens unit with the nonexposure light to facilitate locating the global alignment marks of the semiconductive wafer during the global wafer alignment operation.